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Note 2:deal.II is an
actively developed library, and in the course of this
development we occasionally deprecate and remove
functionality. In some cases, this implies that we also
change tutorial programs, but the nature of videos is that
this is not reflected in something that may have been
recorded years ago. If in doubt, consult
the current version of the tutorial.

Lecture 17.25 gave us a way to estimate or approximate the size of the
error between the exact and discrete solutions on each cell. What
remains for a viable mesh refinement criterion is a rule that helps us
decide which cells to refine based on these indicators. These will
presumably be the cells with the largest indicators, but it's not
quite clear a priori how many of these cells we shall refine.
This lecture introduces different refinement criteria – in
particular the "fixed fraction" or "bulk marking" strategy, and the
"fixed number" strategy. It also touches on a variation used for time
dependent problems. Finally, I talk about two different viewpoints on
what it means to create an adaptive mesh: one being to vastly
increase accuracy at little additional cost, the other being to vastly
reduce the computational cost at little loss of accuracy.